The present invention relates to suspension systems for motor vehicles, and more particularly to a system for measuring the settling of magnetorheological fluids in a suspension damper.
A suspension damper, commonly referred to as a shock absorber, reduces the amplitude of resilient suspension excursions between a sprung mass and an unsprung mass of a motor vehicle by converting into work a fraction of the kinetic energy of the sprung mass. Typically, a suspension damper includes a fluid-filled cylinder tube connected to the unsprung mass, a piston in the cylinder tube connected by a rod to the sprung mass, and valves on the piston which throttle fluid flow across the piston during compression and rebound strokes of the suspension damper attributable to relative suspension excursions between the sprung and unsprung masses.
Some dampers use magnetorheological (MR) fluids, which have been found to have desirable electro-magnetomechanical interactive properties. MR fluids that comprise suspensions of magnetic particles such as iron or iron alloys in a fluid medium have flow characteristics that can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field due to alignment of the magnetic particles to form a structure that resists deformation.
MR fluids used in dampers can settle, i.e. the iron may separate from the liquid and form a sediment at the bottom of the damper. If the settling is severe and the iron sediment is hard-packed, then the performance of the damper may be compromised. Many settling tests have been used in order to predict the behavior of an MR fluid in a damper. These include quiescent settling in a calibrated cylinder, and settling after the fluid was heated and sheared in a mixer. U.S. Pat. No. 5,645,752 discusses the use of ASTM tests D869 and D1309 originally intended to measure settling of paints. U.S. Pat. No. 6,203,717 discloses a settling test in which MR fluid is repeatedly thermally cycled, and the degree of settling is evaluated using a penetrometer test. Both of these patents and the ASTM tests they discuss are hereby incorporated by reference.
None of these tests adequately predict the settling behavior of MR fluid in a damper.
The present invention is a method of testing magnetorheological fluid. The method comprises cycling the fluid at an elevated temperature, allowing the fluid to stand, performance testing the fluid, cycling the fluid at superimposed frequencies, and noting settling characteristics of the fluid.
Accordingly, it is an object of the present invention to provide a method of the type described above which distinguishes between MR fluids that settle hard and those that do not.
Another object of the present invention is to provide a method of the type described above that accounts for the fact that the rate and tendency of MR fluid sedimentation may vary with the amount of durability testing to which a damper has been exposed.
Still another object of the present invention is to provide a method of the type described above that tests the damper and fluid system for sedimentation tendencies at relatively frequent and narrow time intervals with respect to the anticipated overall life of a damper.
These and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawing. The detailed description and drawing are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.